1. Field of the Invention:
The invention relates to a circuit for reducing peak current in dependence on or as a function of line voltage in a blocking oscillator-switched power supply, including a transformer having a primary winding and a control winding, the primary winding being connected in series with an electronic switch, and the control winding supplying a control circuit for controlling the electronic switch, the control circuit including a pulse processing stage and a start-up stage.
2. Description of Related Art:
Blocking oscillator-switched power supplies which are suited for a circuit of the type under discussion are known, for instance, from German Published, Non-Prosecuted Applications DE-OS Nos. 33 47 930 and 33 12 209, the latter corresponding to U.S. Pat. No. 4,593,347. Such blocking oscillator-switched power supplies have a wide control range for a-c line voltage from 90 to 270 V. Besides the constance of the secondary voltage in the case of secondary load variations and primary network voltage variations, protection against overloads and short circuits is to be assured over the entire voltage range.
Protective measures known to date are accomplished either through a voltage monitoring system on the secondary side or through current monitoring on the primary side. In order to obtain the necessary useful power, the current limit must be larger than the maximum current obtained for the minimum voltage, due to the magnetic saturation of the transformer of the switched power supply. If a control range in the order of the voltage limits of 90 to 270 V given above is to be assured, a voltage ratio of 3:1 is obtained, which results in the correspondingly high volume of the power limit. The maximum current value therefore leads to a considerable overload over the entire voltage range, which in individual cases can even lead to destruction. This can only be counteracted with certainty by providing a greatly overdesigned transformer and by tolerating poor efficiency at low voltages. In addition, other components must be overdesigned for the protection of such a blocking oscillator-switched power supply.